This invention relates to liquid heating vessels, and particularly to the control of the electric heating element provided in such vessels. The invention is particularly concerned with electric kettles and more specifically to electric kettles giving the user the capability of selecting a desired water temperature.
The advantages of enabling water in a kettle or coffee maker to be heated to a temperature lower than the boiling point are well known. For example, it is accepted that a water temperature of 80.degree. to 90.degree. is most appropriate for making coffee. The electric kettle described in WO95/34188 includes a control system which enables the temperature of the water to be maintained at a temperature below boiling point. The vessel described in WO95/34188 uses a bi-metal sensor dedicated to the simmering control and which is responsive to a particular temperature in the vicinity of the bi-metal sensor.
This invention aims to provide a vessel in which a desired temperature may be selected by a user, but which requires a simple arrangement of thermal sensors associated with the heating vessel.
One possible way to enable control of the liquid temperature in a liquid heating vessel is to provide a temperature sensor for the contents of the vessel. However, it is preferred not to require temperature sensors in the interior body of the vessel since electrical connections from the external control circuitry into the body of the vessel are then required and a robust thermal sensor is required. In the case of electric kettles, for example, there has also recently been a shift to flat heating elements with printed heating tracks. This technology opens up the possibility of providing printed thermal sensors adjacent the heating tracks. However, if these thermal sensors are to be responsive to the temperature of liquid in the vessel it is required to space the thermal sensor tracks from the heating tracks, or to provide additional measures to isolate the two sets of tracks. It is therefore desirable to enable control of the liquid temperature based on temperature measurements representing the temperature of the heating element rather than that of the liquid in the vessel.
For example, WO96/25869 discloses an electric kettle having a thick film heating element wherein a temperature sensor is provided which detects the temperature of the heating element track, and temperature samples of the heating track are used to determine when boiling of water in the kettle occurs.